The present invention relates to the reduction of the interior temperature within an enclosure such as a building or a vehicle which is exposed to solar radiation.
Control of solar heat load is an important problem, particularly to the automobile industry. Absorption of excessive amounts of solar radiation by the vehicle surfaces can result in higher interior temperatures, reduced passenger comfort and amenity, accelerated degradation of interior materials, and increase of the requirement for larger air conditioning units. Under extreme static-soak conditions, which can occur in vehicles parked in the hot summer sun, especially in a desert climate, surface temperature within a closed car can reach over 200.degree. F. and the entire thermal mass of the car can be raised to a high temperature.
Increasing the cooling load of the air conditioning unit to ameliorate heat discomfort would go against the trend currently prevailing in the automobile industry. Automobile engines are being downsized to reduce weight and improve fuel economy and are less able to handle the power drain of the larger air conditioners. A recent concern to industry and Government is the role played by automotive air conditioners as a source of chlorofluorocarbons (CFC) released into the atmosphere; increased cooling load will lead to even larger air conditioning units, which will exacerbate this problem. Thus, there is a need for new technologies and passive design solutions which would lead to reduced solar heat loads and allow reduction of air conditioner size. These cooling alternatives would result in reduced CFC emission and increased vehicle fuel efficiency.
The automobile glazing is by far the most important factor contributing to the heat problem due to excessive solar loading. A standard sedan typically has over 20 square feet of window area and the glass areas of a sports model sedan can exceed 30 square feet. Because of this fact, glazing can be responsible for over 70 percent of the solar thermal load buildup in a parked car. This imposes an additional requirement on the coating, i.e., it should not impair vision through the glazing below the legal limit. The Federal law (American Standard Safety Code 216-1938) requires that all glass in passenger automobiles must transmit 70 percent of the visible light weighted to the illuminating source "A." The source "A" is a blackbody radiating at 1,416.degree. C. (2,581.degree. F.). The spectral output of the blackbody at this temperature is weighted more heavily in the yellow and red regions. Therefore, coatings with high transmission in the green-red region and low transmission, i.e., high reflection, in the ultraviolet-blue and infrared regions could satisfy the legal requirements and, at the same time, control a large portion of solar radiation. It is theoretically possible to reflect 72 percent of the total solar radiation by this approach and still be within the legal glass transmission limits.
To show the effect of glazing, numerical calculations of the interior air temperatures for the standard and sports model sedan simulations were discussed in the paper "Effects of Glazing and Ventilation Options on Automobile Air Conditioner Size and Performance," R. Sullivan and S. Selkovitz, Lawrence Berkeley Laboratories, Sep. 30, 1988. The simulations discussed in the paper covered the course of an entire day under soak conditions in Phoenix, Ariz. The analysis was accomplished using a finite difference heat transfer computer simulation program called ESP. This program was developed to analyze convective, conductive, and radiative heat flow in buildings. The simulation were conducted using weather data for a typical June day with the outside temperature increasing from 27.degree. C. (81.degree. F.) at 8 a.m. to a peak of 40.degree. C. (104.degree. F.) at 6 p.m. The incident solar radiation on a horizontal surface peaked at 3 p.m. at a value of 1,050 Wm.sup.-2. The results of these calculations present the interior air temperature variations for two models and for four solar transmittances, 83, 43, 23, and 3 percent. A transmission of 83 percent is essentially transmission of a clear glass; solar transmittance of 23 percent is the closest to an ideal coating which would allow 70 percent transmission of the visible light emitted by a 1,416.degree. C. blackbody. It follows from the paper results that use of reflective glazings would provide a substantial reduction in the interior air temperature as the transmittance is decreased.
A solar control coating which transmits only 36 percent of solar radiation, and yet transmits 70 percent of the visible spectrum emitted by the 1,416.degree. C. blackbody, is described in "Auto Solar Control," P. Young and R. Bernardi, SAE 880050 (1988). The developed film is a stack of thin metal and dielectric layers. The metal is highly reflective in the infrared to reflect heat from the sun and the dielectric layers modify transmission characteristics of the film in the visible, to transmit visible light.
Another coating known as "Sungate" is described in "Sun Stopper," Popular Science, October, 1989, at page 66. This coating consists of a stack of metallic and dielectric films sandwiched between two glass plates. The test reported in this article showed that Sungate glass kept the vehicle interior only between 5.degree. and 10.degree. F. cooler than standard glass. The considerable discrepancy in the interior temperature between computer model predictions and actual test values can be attributed to the "greenhouse" effect which builds up slowly due to the absence of radiative cooling. Due to its slow increase, this effect becomes important and affects more the full-day soak interior temperatures than one-hour temperature values.
The inherent limitations of the Sungate-type glass can be understood from the following simple thermal analysis. The visible solar radiation penetrating through the Sungate glass heats the automobile interior to a temperature in the 150.degree.-200.degree. F. range. At these temperatures the interior emits maximum radiation, according to the blackbody law, in the 8 to 10 .mu.m spectral region. Glass is opaque to this radiation; the glass plate facing the interior fully absorbs this radiation and heats up until its temperature becomes equal to the interior temperature. The interior glass plate cannot dissipate heat radiatively because the path to the outside is blocked by the metallic films comprising the Sungate coating which reflects infrared radiation. Thus, glass can only reradiate heat back into the interior creating the "greenhouse" effect. The only cooling mechanism open to the interior glass is non-radiative through heat conduction to the exterior glass plate. However, conductivity of the metal-dielectric film stack is poor, therefore this mechanism is inefficient. As a result, there is a temperature gradient across the Sungate glass thickness, i.e., the inner surface is hotter than the outer surface.
One approach to reduce the solar heat load of an enclosure (for example, a vehicle or a building) is to coat the outside of the enclosure with reflecting materials which reflect all the solar radiation. However, for the windows of cars or houses, it is definitely not a desirable solution, because then visible light will not be able to pass through the windows. Even for non-window areas, from an esthetic point of view, it is often not desirable to reflect all visible light.
Another known type of control coating is a holographic film which acts as a holographic filter to transmit only the portion of the solar spectrum which is in the visible. The disadvantages of the holographic film are angular dependence, grating effect, and questionable far IR emissivity.
The disadvantage of these films is that they also trap the heat; i.e., the film only slows down the heat build-up, but eventually the heat is trapped inside the enclosure. The visible solar radiation penetrating these films can eventually heat the interior of a vehicle to a temperature in the 150.degree.-200.degree. F. range. At these temperatures, the interior emits maximum radiation, according to the blackbody law, in the 8-10 .mu.m, spectral region. The interior heat cannot dissipate radiatively because the surfaces will reflect infrared radiation. The only other passive cooling mechanism is through conduction. However, most building materials are poor thermal conductors. As a result, the inner surface is hotter than the outside surface.
Similar problems and considerations apply to enclosed building structures. Such structures can be heated by solar radiation to an extent that occupants are uncomfortable, leading to increased use of energy to cool the structure interior.
It is therefore an object of the present invention to provide a radiative cooling means for reducing the temperature inside an enclosure which is exposed to solar radiation.
A further object is to provide a coating which reflects incident solar radiation in the infrared, and yet permits heat within the enclosure to be reradiated to the atmosphere as infrared radiation in the 8-13 .mu.m range.